"How Ussher Arrived at the Year of Creation
Who is Ussher?

Archbishop James Ussher was a highly educated and well-respected historian who devoted his life to defending the Christian faith. Ussher meticulously researched the secular accounts of history and found that the Bible correlated with them. Ussher dedicated several years of his life to compiling a history of the world from creation to AD 70.

He was also...

    a well-respected scholar and theologian
    ordained as a priest in the Anglican church at 20
    a great academic, earning his doctorate at 26
    a meticulous researcher
    one of history's most famous historians
    the author of the unparalleled historical work The Annals of the World
    buried in Westminster Abbey

Now you ask: How did he get 4004 BC?

Answer: He used the chronologies in the Hebrew text of Genesis 5 and 11, together with other Bible passages that we will consider. To simplify the calculations, Ussher ties the chronology to the final deportation of Judah in 584 BC. His detailed calculations cover over 100 pages in the original document."